It's set up as part of a two-part authentication scheme, where you add a person that's permitted to view your Address Book in the Sharing Pane of the Address Book application, and then they subscribe to your Address Book under the File Menu in Address Book. I realize that sounds cumbersome, but it's probably about five clicks, all told. Once you have designated someone to share your addresses, and once they subscribe to your Address Book feed through .Mac, your address book is synced to their address book.
Now, it would be great if I could choose a subset of my large Address Book (say, 500 entries) and make, say, 100 of them share-able, just to share my client list with my business partner, or just to share my personal contacts with my girlfriend so we could handle party invites, but that's not part of the deal right now, at least not natively. You might be able to get away with that sort of thing if there was a library switcher for Address Book along the lines of the one for iPhoto.
Alas, they must leave something for the next update, but this is something I'd like to see sooner, rather than later.
Tom Bridge is a writer and technologist in Northern Virginia.
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